Plastics Materials 7 Episode 8 ppt

Because alcoholysis will cause scission of branched polymers at the points where branching has proceeded via the acetate group, polyviny1 alcohol polymer will have a lower molecular weig

Vapour phase synthesis may be carried out by passing a mixture of acetylene and acetic acid through a reaction tube at 210-215°C Typical catalysts for this reaction are cadmium acetate, zinc acetate and zinc silicate The monomer in each of the above mentioned processes is purified by distillation

Purified monomer is usually inhibited before shipment by such materials as copper resinate, diphenylamine or hydroquinone, which are generally removed before polymerisation The monomer is a sweet-smelling liquid partially miscible with water and with the following properties: boiling point at 760mmHg, 72.5"C; specific gravity at 20"C, 0.934; refractive index nD20, 1.395; vapour pressure at 20°C 90mmHg

In 1953 the Celanese Corporation of America introduced a route for the

production of vinyl acetate from light petroleum gases This involved the

oxidation of butane which yields such products as acetic acid and acetone Two derivatives of these products are acetic anhydride and acetaldehyde, which then

react together to give ethylidene diacetate (Figure 14.2.)

Figure 14.2

Exposure of the ethylidene diacetate to an aromatic sulphonic acid in the presence of five times its weight of acetic anhydride as diluent at 136°C will yield the following mixture: 40% vinyl acetate; 28% acetic acid; 20% acetic anhydride; 4% ethylidene diacetate; 8% acetaldehyde

The latter four products may all be reused after separation

In recent years vinyl acetate has been prepared in large quantities by oxidation

of ethylene If ethylene is passed into a solution of palladium chloride in acetic acid containing sodium acetate, then vinyl acetate, ethylene diacetate and acetaldehyde are produced, the vinyl acetate being obtained in good yields by the reaction shown in Figure 14.3

CH,=CH, + 2CH,COONa + PdC1,

CH, COOH

I OOC CH,

Figure 14.3

The ethylene oxidation process can be carried out in either a liquid or a vapour phase but the latter method is often preferred because it avoids corrosion problems and the use of solvents

A one-stage process for producing vinyl acetate directly from ethylene has also been disclosed In this process ethylene is passed through a substantially anhydrous suspension or solution of acetic acid containing cupric chloride and copper or sodium acetate together with a palladium catalyst to yield vinyl acetate

14.2.2 Polymerisation

Vinyl acetate may be easily polymerised in bulk, solution, emulsion and suspension At conversions above 30%, chain transfer to polymer or monomer may occur In the case of both polymer and monomer transfer two mechanisms

are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the acetate group

H

I -CH, - CH - + -CH,-C

I OOC CH,

I OOC CH,

a suitable colloid-emulsifier system, such as poly(viny1 alcohol) and sodium lauryl sulphate, and a water-soluble initiator such as potassium persulphate Polymerisation takes place over a period of about 4 hours at 70°C The reaction is exothermic and provision must be made for cooling when the batch size exceeds a few litres In order to achieve better control of the process and to

Poly(viny1 alcohol) 389 obtain particles with a smaller particle size, part of the monomer is first polymerised and the rest, with some of the initiator, is then steadily added over

a period of 3-4 hours To minimise the hydrolysis of vinyl acetate or possible comonomers during polymerisation, it is necessary to control the pH throughout reaction For this purpose a buffer such as sodium acetate is commonly employed

14.2.3 Properties and Uses

Poly(viny1 acetate) is too soft and shows excessive 'cold flow' for use in moulded plastics This is no doubt associated with the fact that the glass transition temperature of 28°C is little above the usual ambient temperatures and

in fact in many places at various times the glass temperature may be the lower

It has a density of 1.19g/cm3 and a refractive index of 1.47 Commercial polymers are atactic and, since they do not crystallise, transparent (if free from emulsifier) They are successfully used in emulsion paints, as adhesives for textiles, paper and wood, as a sizing material and as a 'permanent starch' A number of grades are supplied by manufacturers which differ in molecular weight and in the nature of comonomers (e.g vinyl maleate) which are commonly used (see Section 14.4.4)

The polymers are usually supplied as emulsions which also differ in the particle size, the sign of the charge on the particle, the pH of the aqueous phase and in other details

Being an amorphous polymer with a solubility parameter of 19.4 MPa'12, it dissolves in solvents with similar solubility parameters (e.g benzene 6 = 18.8MPa1", chloroform 6 = 19.0MPa'/2, and acetone 6 = 20.4MPa'12

Poly(viny1 alcohol) is thus prepared by alcoholysis of a poly(viny1 ester) and in

practice poly(viny1 acetate) is used (Figure 14.5)

Either methanol or ethanol may be used to effect alcoholysis but the former is often preferred because of its miscibility with poly(viny1 acetate) at room

temperature and its ability to give products of better colour Where methanol is employed, methyl acetate may be incorporated as a second solvent It is also formed during reaction The concentration of poly(viny1 acetate) in the alcohol is usually between 10 and 20%

Either acid or base catalysis may be employed Alkaline catalysts such as caustic soda or sodium methoxide give more rapid alcoholysis With alkaline catalysts, increasing catalyst concentration, usually less than 1% in the case of sodium methoxide, will result in decreasing residual acetate content and this phenomenon is used as a method of controlling the degree of alcoholysis Variations in reaction time provide only a secondary means of controlling the reaction At 60°C the reaction may takes less than an hour but at 20°C complete

‘hydrolysis’ may take up to 8 hours

The use of acid catalysts such as dry hydrochloric acid has been described in the literature but are less suitable when incompletely ‘hydrolysed’ products are desired as it is difficult to obtain reproducible results

Commercial poly(viny1 alcohol) (e.g Gelvatol, Elvanol, Mowiol and Rhodo- viol) is available in a number of grades which differ in molecular weight and in the residual acetate content Because alcoholysis will cause scission of branched polymers at the points where branching has proceeded via the acetate group, poly(viny1 alcohol) polymer will have a lower molecular weight than the poly(viny1 acetate) from which it is made

14.3.1 Structure and Properties

Poly(viny1 acetate) is an atactic material and is amorphous Whilst the structure

of poly(viny1 alcohol) is also atactic the polymer exhibits crystallinity and has essentially the same crystal lattice as polyethylene This is because the hydroxyl groups are small enough to fit into the lattice without disrupting it

The presence of hydroxyl groups attached to the main chain has a number of

significant effects The first effect is that the polymer is hydrophilic and will dissolve in water to a greater or lesser extent according to the degree of

‘hydrolysis’ and the temperature Polymers with a degree of ‘hydrolysis’ in the range of 8 7 4 9 % are readily soluble in cold water An increase in the degree of

‘hydrolysis’ will result in a reduction in the ease of solubility and fully

‘hydrolysed’ polymers are only dissolved by heating to temperatures above 85°C

This anomalous effect is due to the greater extent of hydrogen bonding in the completely ‘hydrolysed’ polymers Hydrogen bonding also leads to a number of other effects, for example, unplasticised poly(viny1 alcohol) decomposes below its flow temperature The polymer also has a very high tensile strength and is very tough Films cast from high molecular weight grades, conditioned to 35%

humidity, are claimed2 to have tensile strengths as high as 180001bf/in2 ( I 25 MPa)

The properties will be greatly dependent on humidity; the higher the humidity, the more the water absorbed Since water acts as a plasticiser there will be a reduction in tensile strength but an increase in elongation and tear strength

Figure 14.6 shows the relationship between tensile strength, percentage

‘hydrolysis’ and humidity

Because of its high polarity, poly(viny1 alcohol) is very resistant to hydrocarbons such as petrol Although the polymer will dissolve in lower alcohol- water mixtures, it does not dissolve in pure alcohols As it is crystalline as well as

The Poly(viny1 acetals) 39 1

DEGREE OF HYDROLYSIS IN o/r

Figure 14.6 Relation between tensile strength and degree of ‘hydrolysis’ for unplasticised poly(viny1

alcohol) film (After Davidson and Sittig’)

highly polar only a few organic solvents, such as diethylenetriamine and triethylenetetramine, are effective at room temperature As might be expected, the hydroxyl group is very reactive and many derivatives have been prepared The polymer may be plasticised by polar liquids capable of forming hydrogen bonds with the hydroxyl groups Glycerin has been used for this purpose

14.3.2 Applications

Poly(viny1 alcohol) is employed for a variety of purposes Film cast from aqueous alcohol solution is an important release agent in the manufacture of reinforced plastics Incompletely ‘hydrolysed’ grades have been developed for water-soluble packages for bath salts, bleaches, insecticides and disinfectants Techniques for making tubular blown film, similar to that used with polyethylene, have been developed for this purpose Moulded and extruded products which combine oil resistance with toughness and flexibility are produced in the United States but have never become popular in Europe Poly(viny1 alcohol) will function as a non-ionic surface active agent and is

used in suspension polymerisation as a protective colloid In many applications

it serves as a binder and thickener is addition to an emulsifying agent The polymer is also employed in adhesives, binders, paper sizing, paper coatings, textile sizing, ceramics, cosmetics and as a steel quenchant

Japanese workers have developed fibres from poly(viny1 alcohol) The polymer is wet spun from warm water into a concentrated aqueous solution of

sodium sulphate containing sulphuric acid and formaldehyde, the latter insolubilising the alcohol by formation of formal groups

14.4 THE POLY(V1NYL ACETALS)

Treatment of poly(viny1 alcohol) with aldehydes and ketones leads to the formation of poly(viny1 acetals) and poly(viny1 ketals), of which only the former

products are of any commercial significance (Figure 14.7)

The products are amorphous resins whose rigidity and softening point depend

on the aldehyde used Poly(viny1 butyral), with the larger side chain, is softer than poly(viny1 formal) Since the reaction between the aldehyde and the hydroxyl groups occurs at random, some hydroxyl groups become isolated and are incapable of reaction A poly(viny1 acetal) molecule will thus contain:

(1) Acetal groups

(2) Residual hydroxyl groups

(3) Residual acetate groups, due to incomplete 'hydrolysis' of poly(viny1 acetate) to poly(viny1 alcohol)

14.4.1 Poly(viny1 formal)

The poly(viny1 acetals) may be made either from poly(viny1 alcohol) or directly from poly(viny1 acetate) without separating the alcohol In the case of poly(viny1 formal) the direct process is normally used

In a typical process, 100 parts of poly(viny1 acetate) are added to a mixture of

200 parts acetic acid and 70 parts water, which has been warmed to about 70°C, and stirred to complete solution Sixty parts of 40% formalin and 4 parts sulphuric acid (catalyst) are added and reaction is carried out for 24 hours at 70°C Water is added to the mixture with rapid agitation to precipitate the granules, which are then washed free from acid and dried

A number of grades of poly(viny1 formal) are commercially available (Formvar, Mowital) which vary in degree of polymerisation, hydroxyl content and residual acetate content

residual hydroxyl content is expressed in terms of poly(viny1 alcohol) content and residual acetate in terms of poly(viny1 acetate) content

The Poly(viny1 acetals) 393

Table 14.1 Influence of structure variables on the properties of poly(viny1 formal)

500 5-6 9.5-13 160-170 88-93

10

69 7-20 1.2-2.0 0.75

10

69 10-50 1.2-2.0 1.1

ASTM test

-

-

- D.569-48T D.648-49T

D.638-41T

- D.638-41T D.256-43T D.570-40T

Various grades of poly(vinyl formal)

350 7-9 9.5-13 140-145 88-93

10

69 10-50 1.0-1.4 1.1

430 5-7 20-27 145-150 75-80

10

69 4-5 0.5-0.7 1.5

350

5 -7 40-50 50-60

-

10

69 3-4 0.4-0.6 1-5

It will be observed that molecular weight has little effect on mechanical properties but does influence the flow temperature

The hydroxyl content of commercial material is kept low but it is to be observed that this has an effect on the water absorption Variation in the residual acetate content has a significant effect on heat distortion temperature, impact strength and water absorption The incorporation of plasticisers has the usual influence on mechanical and thermal properties

The polymer, being amorphous, is soluble in solvents of similar solubility parameter, grades with low residual acetate being dissolved in solvents of solubility parameter between 19.8 and 22 MPa’”

The main application of poly(viny1 formal) is as a wire enamel in conjunction with a phenolic resin For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used Similar grades are used in structural adhesive (e.g Redux) which are also used in conjunction with phenolic resin Poly(viny1 formal) finds some use as a can coating and with wash primers Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices

14.4.2 Poly(viny1 acetal)

Poly(viny1 acetal) itself is now of little commercial importance The material may

be injection moulded but has no particular properties which merit its use It is occasionally used in conjunction with nitrocellulose in lacquers, as a vehicle for wash primers and as a stiffener for fabrics

14.4.3 Poly(viny1 butyral)

As a safety glass interleaver, poly(viny1 butyral) (Butacite, Saflex) is extensively used because of its high adhesion to glass, toughness, light stability, clarity and moisture insensitivity

It also finds miscellaneous applications in textile and metal coatings and in adhesive formulations Where it is to be used as a safety glass interleaver, a very pure product is required and this is most conveniently prepared from

poly(viny1 alcohol) rather than by the direct process from poly(viny1 acetate)

In a typical process 140 parts of fully ‘hydrolysed’ poly(viny1 alcohol) are

suspended in 800 parts of ethanol; 80 parts of butyraldehyde and 8 parts of sulphuric acid are added and the reaction is carried out at about 80°C for 5-6

hours

The solution of poly(viny1 butyral) is diluted with methanol and the polymer precipitated by the addition of water during vigorous agitation The polymer

is then stabilised, washed and dried

Highly ‘hydrolysed’ poly(viny1 alcohol) is normally used as a starting point For safety glass applications about 25% of the hydroxyl groups are left unreacted In this application the polymer is plasticised with an ester such as dibutyl sebacate or triethylene glycol di-2-ethyl butyrate, about 30 parts of plasticiser being used per 100 parts of polymer The compound is then calendered to a thickness of 0.015 in and coated with a layer of sodium bicarbonate to prevent blocking To produce safety glass the film is washed and dried and then placed between two pieces of glass which are then subjected to mild heat and pressure Bulletproof glass is made by laminating together several layers of glass and poly(viny1 butyral) film

Laminated safety glass has now become standard for automobile wind- screens and is used for aircraft glazing

14.5 ETHYLENE-VINYL ALCOHOL COPOLYMERS

If ethylene is copolymerised with vinyl acetate, and the vinyl acetate component ‘hydrolysed’ to vinyl alcohol, a material is produced which is in effect a copolymer of ethylene and vinyl alcohol

The material is produced by Kurardy and Nippon Gohsei in Japan and was also produced up until 1993 by Du Pont Global nameplate capacity has increased from about 30 000 t.p.a early in the 1990s to 60 000 t.p.a at the end

of the millenium The material is commonly referred to in the abbreviated form EVOH but occasionally also as EVAL and EVOL

Certain copolymers of this type have been found to have excellent gas barrier properties, with the dry polymer having an oxygen permeability only about 1 / I 0th that of polyvinylidene chloride Unsurprisingly, the copolymer has

a high moisture absorption and a high moisture vapour transmission rate Where the material is swollen by water, gas permeability is also higher For reasons explained below, the effect of increasing the ‘vinyl alcohol’ content in EVOH is quite different to that of increasing the vinyl acetate content in EVA In the case of ethylene-vinyl acetate (EVA) copolymers, increasing the vinyl acetate content up to about 50% makes the materials less crystalline and progressively more flexible and then rubbery In the range 40-70% vinyl acetate content the materials are amorphous and rubbery, whilst above 70% the copolymers become increasingly rigid and brittle

Commerical grades of EVOH typically have ‘vinyl alcohol’ contents in the range 56-71%, but in contrast to the corresponding EVA materials these copolymers are crystalline Furthermore, an increase in the ‘vinyl alcohol’

content results in an increase in such properties as crystalline melting point,

tensile strength and tensile modulus together with a decrease in oxygen permeability This is a reflection of the fact that the ethylene and vinyl alcohol units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1)

Poly(viny1 cinnamate) 395

Table 14.2 Typical properties of EVOH copolymers (For purposes of comparison the grades selected all have a MFI (2.16kg, 190OC) of 1.7-1.8 Grades with other MFI values are also available)

3900 0.23 0.8

100-200

3700 0.30

Some typical properties of some commercial EVOH polymers (Soamol-

Nippon Gohsei) are given in Table 14.2

As is to be expected, the table shows that as the humidity is increased, causing swelling and an increase in the interchain separation, so the oxygen permeability increases Also, as expected, the percentage increase is greater the higher the vinyl alcohol content

Because of the excellent gas barrier properties, EVOH is of interest as a packaging material However, because of its high water absorption it is usually used as an internal layer in a co-extruded film, sheet, bottle or tube For example, the system HDPE-EVOH-EVA may be used as a barrier film for packaging cereals, and the system polystyrene-EVOH-polystyrene for packag- ing coffee and cream, whilst the system polystyrene-EVOH-polyethylene has the additional advantage of heat sealability

In the case of EVOH being used as an interlayer with polyethylene or polystyrene, it is necessary to use additional adhesive layers such as

an ethylene-vinyl acetate-maleic anhydride terpolymer (e.g Orevac- Atochem)

While EVOH is of interest primarily for food packaging applications attention

is now being turned to non-food outlets such as automotive fuel tanks, floor heating pipes and toothpaste tubes

14.6 POLY(V1NYL CINNAMATE)

Poly(viny1 cinnamate) is not used in the traditional areas of plastics technology but its ability to cross-link on exposure to light has led to important applications

in photography, lithography and related fields as a photoresist

The concept of a Photoresist is of great antiquity and has a number of features

of interest relating to plastics In Ancient Egypt mummies were wrappted in linen cloths dipped in a solution of oil of lavender containing high molecular mass bituminous material (Chapter 30) which was known variously as Syrian Asphalt

or Bitumen of Judea On exposure to light the product hardened and became insoluble The evidence is that some form of cross-linking occurred

At the beginning of the nineteenth century, an amateur Egyptologist, J

Nictphore Niepce, became interested in the process and in 1822 he adapted it to produce the first permanent photograph It also played an important role in the development of lithography In essence surfaces exposed to light become insoluble and cannot be removed by solvents whilst unexposed surfaces remain soluble and can be so removed This is the concept of a negative photoresist (There also exist positive photoresists, including some phenolic resins, which become more soluble on exposure to light) Today photoresists are used in the fabrication of solid-state electronic components and integrated circuits and poly(vin1y cinnamate) is one of the longest established materials of this type

As with poly(viny1 alcohol), poly(viny1 cinnamate) is prepared by chemical

modification of another polymer rather than from ‘monomer’ One process is to treat poly(viny1 alcohol) with cinnamoyl chloride and pyridine but this is rather slow Use of the Schotten Baumann reaction will, however, allow esterification

to proceed at a reasonable rate In one example4 poly(viny1 alcohol) of degree of

polymerisation 1400 and degree of saponification of 95% was dissolved in water

To this was added a concentrated potassium hydroxide solution and then cinnamoyl chloride in methyl ethyl ketone The product was, in effect a vinyl alcohol-vinyl cinnamate copolymer Figure 14.8)

+ @-CH=CH.COCI

Cinnamoyl Chloride

Figure 14.8

To make a photoresist poly(viny1 cinnamate), or a high vinyl cinnamate

copolymer, is dissolved in a solvent such as methylene dichloride and the

solution is coated uniformly over the substrate by a process such as spin casting After evaporation of the solvent a masking material (which in the case of a simple demonstration could be a paper clip) is placed on the resist and the assembly is exposed to ultraviolet light The exposed surfaces are then insolubilised After exposure the mask is removed and soluble matter dissolved

in a solvent such as cellosolve acetate and this exposes the substrate in the shape

of the mask This may then be etched or otherwise treated as required By the use

of appropriate sensitisers such as, 1,2-benzanthraquinone or Michler ’s ketone the

cross-linking may be brought about by visible light The cross-linking is believed

to involve the production of a four-membered cyclobutane ring (Figure 14.9)

Figure 14.9

Bibliography 397 14.7 OTHER ORGANIC VINYL ESTER POLYMERS

Polymers from many other vinyl esters, such as vinyl propionate, vinyl caproate, vinyl benzoate, vinyl stearate and vinyl laurate, have been prepared on a commercial scale As is to be expected, increasing the length of the side chain reduces the softening point of the polymer so that polymers similar in many ways

to the higher acrylates and methacrylates may be obtained It is also of interest

to note that, as with acrylates and methacrylates, the glass transitions of the polymers go through a minimum with about twelve carbon atoms in the side chain, side-chain crystallisation becoming important with higher homologues

Of the higher vinyl ester homopolymers only poly(viny1 propionate) is currently believed to be of commercial value, being marketed as Propiofan (BASF) for surface coating application where greater alkali resistance is possible than with the normal vinyl acetate based copolymers

Whilst vinyl acetate is reluctant to copolymerise it is in fact usually used today

in copolymers Two of particular interest to the plastics industry are ethylene- vinyl acetate (Chapter 11) and vinyl chloride-vinyl acetate copolymers (Chapter

12) In surface coatings internal plasticisation to bring the Tg to below ambient temperatures and thus facilitate film forming is achieved by the use of ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and dialkyl maleates and fumarates

References

1 HORN.O., Chem Ind., 1749 (1955)

2 DAVIDSON, R.L., and SITTIG, M., Water-soluble Resins, Reinhold, New York (1962)

3 FITZHUGH, A.F., and LAVIN, E., J Electrochem Soc., 100 (8), 351 (1953)

4 DELZENNE, G.A., Encyclopaedia of Polymer Science and Technology, Supplement, Vol 1, p.401,

Wiley, New York (1976)

Bibliography

General

KAINER, F., Polyvinylakohole, Enka, Stuttgart (1949)

LEONARD, E c (Ed.), Vinyl and Diene Monomers (High Polymers Series Vol 24) Wiley-Interscience, SCHILDKNECHT, c.E., Vinyl and Related Polymers, John Wiley, New York (1952)

Encyclopaedia of Polymer Science and Technology Vols 14 and 15, Wiley-Interscience, New York

DAVIDSON R.L., and SITI-IG, M., Water-soluble Resins (2nd Ed.), Reinhold, New York (1968)

RNCH, C.A (Ed.), Polyvinyl alcohol: Properties and Applications, Wiley New York (1973)

PRITCHARD, J.c., Poly(Viny1 alcohol): Basic Properties and Uses, Macdonald, London (1970) Properties and Applications of Polyvinyl Alcohol (SCI Monograph No 30), Society of the Chemical

Polyvinyl Acetals

PITZHUGH, A.F and LAWN, E., J Electrochem Soc., 100 (8), 351 (1953)

PLATZER N Mod Plastics, 28, 142 (1951)

New York (1971)

Industry, London (1968)

15

Acrylic Plastics

15.1 INTRODUCTION

Poly(methy1 methacrylate) (Figure 15.1, I) is, commercially, the most important

member of a range of acrylic polymers which may be considered structurally as derivatives of acrylic acid (11)

This family includes a range of polyacrylates (111), polymethacrylates (IV) and the important fibre-forming polymer, polyacrylonitrile (V)

Methyl, ethyl and allyl acrylate were first prepared in 1873 by Caspary and Tollens,’ and of these materials the last was observed to polymerise In 1880 Kahlbaum2 reported the polymerisation of methyl acrylate and at approximately the same time Fittig”‘ found that methacrylic acid and some of its derivatives readily polymerised

In 190 1 Otto Rohm reported on his studies of acrylic polymers for his doctoral dissertation His interest in these materials, however, did not cease at this stage and eventually in 1927 the Rohm and Hass concern at Darmstadt, Germany commenced limited production of poly(methy1 acrylate) under the trade names

Figure 15.1

398

Introduction 399 Acryloid and Plexigum These were soft gummy products of interest as surface coatings rather than as mouldable plastics materials About 1930 R Hill in England and W Bauer in Germany independently prepared poly(methy1 methacrylate) and found it to be a rigid, transparent polymer, potentially useful

as an aircraft glazing material.5

The first methacrylic esters were prepared by dehydration of hydroxyisobutyric esters, prohibitively expensive starting points for commercial synthesis In 1932

J W C Crawford6 discovered a new route to the monomer using cheap and readily available chemicals-acetone, hydrocyanic acid, methanol and sulphuric acid- and it is his process which has been used, with minor modifications, throughout the world Sheet poly(methy1 methacrylate) became prominent during World War I1

for aircraft glazing, a use predicted by Hill in his early patents, and since then has found other applications in many fields

Examples of commercial poly(methy1 methacrylate) sheet are Perspex (ICI), Oroglas and Plexiglas (Atoglas) Poly(methy1 methacrylate) moulding powders include Diakon (ICI), Acry-ace (Fudow Chemical Co., Japan), Lucite (Du Pont) and Vedril (Montecatini)

In addition to poly(methy1 methacrylate) plastics and polyacrylonitrile fibres, acrylic polymers find widespread use First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known ‘super-glues’ are also made from acrylic polymers

During the 1970s there was considerable interest for a time in copolymers with

a high acrylonitrile content for use as barrier resins, i.e packaging materials with low permeability to gases Problems associated with free acrylonitrile have, however, led to the virtual disappearance of these materials from the market Other developments in recent years have been the appearance of tough and heat-resistant materials closely related to poly(methy1 methacrylate) and to interesting cross-linked polymers Amongst these are the so-called hydrophilic polymers used in the making of soft contact lenses

Today a very wide range of acrylic materials is available with a broad property spectrum The word acrylic, often used as a noun as well as an adjective in everyday use, can mean quite different things to different people In the plastics industry it is commonly taken to mean poly(methy1 methacrylate) plastics, but the word has different meanings, to the fibre chemist and to those working in the paint and adhesives industries Unless care is taken this may be a source of some confusion

As with other major plastics materials, there is at present little use of the IUPAC systematic nomenclature, which is based on the nature of the repeating unit rather thafi the monomer used The following names may, however, be noted:

15.2 POLY(METHYL METHACRYLATE)

15.2.1 Preparation of Monomer

This successful commercial utilisation of poly(methy1 methacrylate) is due in no

small measure of the process of producing the monomer from acetone developed

by Crawford of IC1 which enabled the polymer to be produced at a competitive price Some details of the process as operated by the Rohm and Hass Company

of Philadelphia have been d i ~ c l o s e d ~

Acetone is first reacted with hydrogen cyanide to give acetone cyanohydrin (Figure 15.2)

CH,

CH,

Figure 15.2

The cyanohydrin is then treated with 98% sulphuric acid in a cooled hydrolysis

kettle to yield methacrylamide sulphate (Figure 15.3)

The sulphate is not isolated from the reaction mixture, which passes into an

esterification kettle and reacts continously with methanol (Figure 15.4)

is based on the two-stage oxidation of isobutylene or t-butyl alcohol to methacrylic acid, which is then separated and esterified Figure 1 5 5 ~ )

The monomer is a mobile liquid with a characteristic sweet odour and with the following properties:

Free-radical polymerisation techniques involving peroxides or azodi- isobutyronitrile at temperatures up to about 100°C are employed commercially The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas

It has been observed that in the polymerisaton of methyl methacrylate there is

an acceleration in the rate of conversion after about 20% of the monomer has been converted The average molecular weight of the polymer also increases during polymerisation It has been shown that these results are obtained even under conditions where there is a negligible rise in the temperature ( 4 ° C ) of the reaction mixture

The explanation for this effect (known variously as the gel effect, Tromsdorff effect or auto-acceleration effect) is that the chain termination reaction slows down during conversion and, as can be seen by reference to equations (2.5) and (2.6), a decrease in the termination rate constant leads to

an increase in both overall rate and molecular weight The reason for the drop

in termination rate is that as the reaction mixture becomes more viscous the radical ends of the polymer chains find increased difficulty in diffusing towards each other, leading to the important mutual termination reaction Small monomer molecules on the other hand find little difficulty in diffusion at moderate conversion so that propagation reactions are relatively little affected, until the material becomes semi-solid, when the propagation rate constant also decreases It is of interest to note that the gel effect may be induced by the addition of already formed poly(methy1 methacrylate) or even another polymer such as cellulose tripropionate because such additions increase the viscosity of the system

The auto-acceleration effect appears most marked with polymers that are insoluble in their monomers In these circumstances the radical end becomes entrapped in the polymer and termination reactions become very difficult It has been suggested that, in thermodynamic terms, methyl methacrylate is a relatively poor solvent for poly(methy1 methacrylate) because it causes radicals to coil while in solution The termination reaction is then determined by the rate at which the radical ends come to the surface of the coil and hence become available for mutual termination

Bulk polymerisation is extensively used in the manufacture of the sheet and to a

lesser extent rod and tube In order to produce a marketable material it is important to take the following factors into account:

(1) The exotherm developed during cure

(2) The acceleration in conversion rate due to increasing viscosity

( 3 ) The effect of oxygen

(4) The extensive shrinkage in conversion from monomer to polymer (-20%) (5) The need to produce sheet of even thickness

(6) The need to produce sheet of constant quality

(7) The need to produce sheet free from impurities and imperfections

Poly(methy1 methacrylate) 403

In order to reduce the shrinkage in the casting cell, and also to reduce problems

of leakage from the cell, it is normal practice to prepare a ‘prepolymer’ In a typical process monomer freed from inhibitor is heated with agitation for about

8 minutes at 90°C with 0.5% benzoyl peroxide and then cooled to room temperature Plasticiser, colouring agents and ultraviolet light absorbers may be incorporated at this stage if required The resulting syrup, consisting of a solution

of polymer in monomer, is then filtered and stored in a refrigerator if it is not required for immediate use The heating involved in making the prepolymer may also be of assistance in removing oxygen dissolved in the monomer

The preparation of a prepolymer requires careful control and can be somewhat difficult in large-scale operations An alternative approach is to prepare a syrup

by dissolving some polymer in the monomer and adding some peroxide to the mixture As in the case of a prepolymer syrup, such a syrup will cause less shrinkage on polymerisation and fewer leakage problems

Acrylic sheet is prepared by pouring the syrup into a casting cell This consists

of two plates of heat-resistant polished glass provided with a separating gasket round the edges The gasket commonly consists of a hollow flexible tube made from a rubber, or from plasticised poly(viny1 alcohol) The cell is filled by opening up the gasket at a corner or edge and metering in the syrup, care being taken to completely fill the cell before closing up the gasket The cell is held together by spring-loaded clamps or spring clips so the plates will come closer together as the reacting mixture shrinks during polymerisation This technique will enable the sheet to be free of sink marks and voids

It is important to use rigid glass sheet and to apply pressure to the plates in such a manner that they do not bow out as this would lead to sheet of uneven thickness

The filled cells are then led through a heating tunnel In a typical system the time to pass through the tunnel is about 16 hours For the first 14 hours the cell passes through heating zones at about 40°C Under these conditions polymer- isation occurs slowly Any acceleration of the rate due to either the rise in temperature through the exothermic reaction or due to the viscosity-chain termination effect will be small It is particularly important that the temperature

of any part of the syrup is not more than 100°C since this would cause the monomer to boil By the end of this period the bulk of the monomer has reacted and the cell passes through the hotter zones After 15 hours (total time) the cell

is at about 97”C, at which temperature it is held for a further half-hour The sheet

is then cooled and removed from the cell In order to reduce any internal stresses the sheet may be annealed by heating to about 140°C and, before being dispatched to the customer, the sheet is masked with some protective paper using gelatine or, preferably, with a pressure-sensitive adhesive

When casting large blocks, the exothexm problem is more severe and it may be necessary to polymerise inside a pressure vessel and thus raise the boiling point

of the monomer

In order to compensate for shrinkage, special techniques are required in the manufacture of rod In one process, vertical aluminium tubes are filled with syrup and slowly lowered into a water bath at 40°C As the lowest level of syrup polymerises, it contracts and the higher levels of syrup thus sink down the tube, often under pressure from a reservoir of syrup feeding into the tubes

Acrylic tubes may be prepared by adding a calculated amount of syrup to an aluminium tube, sealing both ends, purging the air with nitrogen and then rotating horizontally at a constant rate The whole assembly is heated and the

syrup polymerises on the wall of the rotating tube The natural shrinkage of the material enables the casting to be removed quite easily

An interesting modification of the sheet casting process is the band polymerisation process due to Swedlow.8 In this process a monomer/polymer syrup is polymerised between steel bands which pass through heating zones and which are spaced according to the sheet thickness required Whilst there may be some economic attraction of the process in some countries with high labour costs the quality of the product is generally inferior to that of cell-cast sheet Furthermore, where lower optical qualities are tolerable extruded sheet is generally cheaper to produce The process, as with the cast cell process, does however allow for the possibility of cross-linked polymer sheet that cannot easily

be produced by extrusion processes

Suspension polymerisation

The average molecular weight of most bulk polymerised poly(methy1 methacry- lates) is too high to give a material which has adequate flow properties for injection moulding and extrusion

By rolling on a two-roll mill the molecular weight of the polymer can be greatly reduced by mechanical scission, analogous to that involved in the mastication of natural rubber, and so mouldable materials may be obtained However, bulk polymerisation is expensive and the additional milling and grinding processes necessary make this process uneconomic in addition to increasing the risk of contamination

As a result the suspension polymerisation of methyl methacrylate was developed to produce commercial material such as Diakon made by ICI Such a polymerisation can be carried out rapidly, usually in less than an hour, because there is no serious exotherm problem

There is, however, a problem in controlling the particle size of the beads formed and further in preventing their agglomeration, problems common to all suspension-type polymerisations The particle size of the beads is determined by the shape and size of the reactor, the type and rate of agitation and also the nature

of suspending agents and protective colloids present Suspending agents used include talc, magnesium carbonate and aluminium oxide whilst poly(viny1 alcohol) and sodium polymethacrylate are among materials used as protective colloids

In one process described in the literature’ one part of methyl methacrylate was agitated with two parts of water and 0.2% benzoyl peroxide was employed as the catalyst Eight to 18 g of magnesium carbonate per litre of reactants were added, the lower amount being used for larger beads, the larger for small beads The reaction temperature was 80°C initially but this rose to 120°C because of the exothermic reaction Polymerisation was complete in about an hour The magnesium carbonate was removed by adding sulphuric acid to the mixture The beads were then filtered off, carefully washed and dried

Other additives that may be incorporated include sodium hydrogen phosphates

as buffering agents to stabilise that pH of the reaction medium, lauryl mercaptan

or trichlorethylene as chain transfer agents to control molecular weight, a lubricant such as stearic acid and small amounts of an emulsifier such as sodium lauryl sulphate

The dried beads may be supplied as injection moulding material without further treatment or they may be compounded with additives and granulated

Poly(methy1 methacrylate) 405

15.2.3 Structure and Properties

Commercial poly(methy1 methacrylate) is a transparent material, and micro- scopic and X-ray analyses generally indicate that the material is amorphous For this reason the polymer was for many years considered to be what is now known

as atactic in structure It is now, however, known that the commercial material is more syndiotactic than atactic (On one scale of assessment it might be considered about 54% syndiotactic, 37% atactic and 9% isotactic Reduction in the temperature of free-radical polymerisation down to -78°C increases the amount of syndiotacticity to about 78%)

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene Differences in the Tg’s of commercial polymers (approx 104”C), syndiotactic polymers (approx 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups

In consequence of a T g of 104°C with its amorphous nature, commercial poly(methy1 methacrylate) is thus a hard transparent plastics material in normal conditions of use

Because the polymer is polar it does not have electrical insulation properties comparable with polyethylene Since the polar groups are found in a side chain these are not frozen in at the Tg and so the polymer has a rather high dielectric constant and power factor at temperatures well below the Tg (see also Chapter 6) This side chain, however, appears to become relatively immobile at about 20”C, giving a secondary transition point below which electrical insulation properties are significantly improved The increase in ductility above 40°C has also been associated with this transition, often referred to as the

@transition

The solubility of commercial poly(methy1 methacrylate) is consistent with that expected of an amorphous thermoplastic with a solubility parameter of about 18.8 MPa’” Solvents include ethyl acetate (6 = 18.6), ethylene dichloride (6 =

20.0), trichloroethylene (6 = 19), chloroform (6 = 19) and toluene (6 = 20), all

in units of MPa’/* Difficulties may, however, occur in dissolving cast poly(methy1 methacrylate) sheet because of its high molecular weight

Since the polymers are unbranched (apart from the methyl and methacrylate side groups) the main difference between uncompounded commercial grades is

in the molecular weight

Cast material is stated to have a number average molecular weight of about lo6 Whilst the T g is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx 170°C) There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped For injection moulding and extrusion much lower molecular weight materials are employed Such polymers have a reasonable melt viscosity but marginally lower heat distortion tem- peratures and mechanical properties

15.2.4 General Properties of Poly(methy1 methacrylate)

As indicated in the previous section poly(methy1 methacrylate) is a hard, rigid,

transparent material Commercial grades have extremely good weathering resistance compared with other thermoplastics

Table 15.1 Some properties of methyl methacrylate polymers

Izod impact strength

Vicat softening point

lo3 Ibf/in'

MPa Io" Ibf/in2 MPa lo3 Ibf/in*

- D.792 D.638

-

-

- D.785

D.570 (B.S.)

2782 D.648

Acylic sheet*

-106 1.19

-430 (3000) -20 -400 (2750) M.lOO (140)

-60 000 1.18 10.5 (72.5) -350 (2400) -18 -400 (2750) MI03

-

2-3 0.3 0.40 109-112 85-95

1.49

>io17 3.1

Copolymer$

- 1.17

- -400 (2750) -18 (130)

* Persrx (ICI) t Diakon M (ICI) $ Astente (ICI) (withdrawn)

The properties of three types of poly(methy1 methacrylate) (sheet based on

high molecular weight polymer, lower molecular weight injection moulding material and a one-time commercial copolymer) are given in Table 15.1

As might be expected of a somewhat polar thermoplastics material,

mechanical, electrical and other properties are strongly dependent on tem- perature, testing 'rate' and humidity Detailed data on the influence of these variables have been made available by at least one manufacturer and the following remarks are intended only as an illustration of the effects rather than

as an attempt at providing complete data

Figure 15.6 shows the considerable temperature sensitivity of the tensile

strength of acrylic sheet whilst Figure 15.7 shows how the fracturing stress decreases with the period of loading Mouldings from acrylic polymers usually show considerable molecular orientation It is observed that a moulding with a high degree of frozen-in orientation is stronger and tougher in the direction parallel to the orientation than in the transverse direction

Poly(methy1 methacrylate) is recognised to be somewhat tougher than polystyrene (after consideration of both laboratory tests and common experience) but is less tough than cellulose acetate or the ABS polymers It is superior to untreated glass in terms of impact resistance and although it cracks, any fragments formed are less sharp and jagged than those of glass and, normally consequently less harmful However, oriented acrylic sheet such as may result

Poly(methy1 methacrylate) 407

Figure 15.6 Effect of temperature on tensile strength of acrylic sheet (Perspex) at constant rate of

strain (0.44% per second) (Reproduced by permission of ICI)

PERIOD OF LOADING IN 5CC

Figure 15.7 Effect of period of loading on fracturing stress at 25°C of acrylic sheet (Perspex)

(Reproduced by permission of ICI)

from double curvature shaping shatters with a conchoidal fracture and fragments and broken edges can be quite sharp Although it is harder than most other thermoplastics the scratch resistance does leave something to be desired Shallow scratches may, however, be removed by polishing

The optical properties of poly(methy1 methacrylate) are particularly important Poly(methy1 methacrylate) absorbs very little light but there is about 4%

reflection at each polymer-air interface for normal incident light Thus the light transmission of normal incident light through a parallel sheet of acrylic material free from blemishes is about 92% The influence of the wavelength of light on

transmission is shown in Figure 15.8

The interesting property of total internal reflection may be conveniently exploited in poly(methy1 methacrylate) Since the critical angle for the polymer-

air boundary is 42°C a wide light beam may be transmitted through long lengths

of solid polymer Light may thus be ‘piped’ round curves and there is little loss where the radius of curvature is greater than three time the thickness of the sheet

or rod Scratched and roughened surfaces will reduce the internal reflection This

is normally undesirable but a roughened or cut area can also be deliberately

WAVELENGTH IN 1

Figure 15.8 Light transmission of acrylic polymer (i in thick moulded Diakon Parallel light beam

normally incident on surface) (Reproduced by permission of ICI)

incorporated to ‘let out’ the light at that point The optical properties of

poly(methy1 methacrylate) have been exploited in the development of optical fibres

Poly(methy1 methacrylate) is a good electrical insulator for low-frequency work, but is inferior to such polymers as polyethylene and polystyrene, particularly at high frequencies The influence of temperature and frequency on

the dielectric constant is shown in Figure 15.9

Figure 15.9 The variation of dielectric constant with temperature and frequency (Perspex) (the lines

join points of equal dielectric constant) (Reproduced by permission of ICI)

Figure 15.IO The dependence of apparent volume resistivity on time of polarisation of acrylic

polymer (Perspex) (Reproduced by permission of ICI)

Poly(methy1 methacrylate) 409

The apparent volume resistivity is dependent on the polarisation time (Figure

15.10) The initial polarisation current is effective for some time and if only a

short time is allowed before taking measurements low values for volume resistivity will be obtained

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methy1 methacrylate) is soluble in a number of solvents with similar solubility parameters Some examples were given in the previous section The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions A number of organic materials although not solvents may cause crazing and cracking, e.g aliphatic alcohols

15.2.5 Additives

Poly(methy1 methacrylate) may be blended with a number of additives Of these the most important are dyes and pigments and these should be stable to both processing and service conditions Two particular requirements are, firstly, that when used in castings they should not affect the polymerisation reaction and, secondly, that they should have good weathering resistance

Plasticisers are sometimes added to the polymer, dibutyl phthalate being commonly employed in quantities of the order of 5% Use in moulding powders will enhance the melt flow but somewhat reduce the mechanical properties of the finished product

Further improvement in light stability may be achieved by addition of small quantities of ultraviolet absorbers Typical examples include phenyl salicylate,

2,4-dihydroxybenzophenone, resorcinol monobenzoate, methyl salicylate and stilbene

15.2.6 Processing

In commercial practice three lines of approach are employed in order to produce articles from poly(methy1 methacrylate) They are:

(1) Processing in the melt state such as by injection moulding and extrusion

( 2 ) Manipulation of sheet, rod and tube

(3) The use of monomer-polymer doughs

There are a number of general points to be borne in mind when processing the polymer in the molten state which may be summarised as follows:

(1) The polymer granules tend to pick up moisture (up to 0.3%) Although most commercial grades are supplied in the dry condition, subsequent exposure before use to atmospheric conditions will lead to frothy mouldings and extrudates, owing to volatilisation of the water in the heating cylinders Particular care should be taken with reground scrap

( 2 ) The melt viscosities at the processing temperatures employed are con- siderably higher than those of polystyrene, polyethylene and plasticised PVC This means that the equipment used must be robust and capable of generating high extrusion and injection pressures The injection moulding of poly(methy1 methacrylate) (PMMA) has been made much easier by the widespread use of the reciprocating screw in-line injection moulding

to remove unwanted moisture and even monomer which has been produced

by depolymerisation of the polymer because of overheating

The melt viscosity is more sensitive to temperature than that of most

thermoplastics (Figure 15.11) and this means that for accurate, consistent and reproducible results, good temperature control is required on all equipment (3) Since the material is amorphous the moulding shrinkage is low and normally less than 0.008 cm/cm

When heated above the glass transition temperature (-lOO"C), acrylic sheet from high molecular weight polymer becomes rubbery The rubbery range

extends for 60°C Further raising of the temperature causes decomposition rather

than melting The reasonably wide rubbery range, c.f cellulose acetate, high- impact polystyrene and polyethylene, enables the sheet to be heated in ovens rather than having to be heated while clamped to the shaping apparatus Poly(methy1 methacrylate) is not widely suitable for normal vacuum forming operations since the modulus of the material in the rubbery state is too great to

Poly(methy1 methacrylate) 4 11 allow shaping of fine detail simply by atmospheric pressure As a result a large number of techniques have been devised using air pressure, mechanical pressure,

or both in combination, and sometimes also involving vacuum assistance The use of monomer-polymer doughs has been largely confined to the production of dentures A plaster of Paris mould is first prepared from a supplied impression of the mouth Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water After polymerisation, which usually takes less than half an hour, the mould

is cooled and opened This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited

A novel technique has been developed for the manufacture of tiles and sanitary ware A dispersion of a ground sand in methyl methacrylate monomer is prepared with a solids content of about 72% by weight The particle size is such that the dispersion has reasonable stability but is pourable When required for use the dispersion is blended with a free-radical initiator, usually based on a peroxide, and fed into metal moulds heated to about 70°C As the monomer polymerises there is

a shrinkage of about 1 1 % by volume and this is compensated through a reduction in the volume of the mould cavity, with one mould half moving towards the other and into the other like a piston in a cylinder The polymerised products have a remarkably good finish, are virtually stress free and have considerable flexibility

in part design Casting dispersions are available from IC1 as Asterite (reviving a name at one time used for a now-obsolete acrylic copolymer)

15.2.7 Applications

The major uses of poly(methy1 methacrylate) arise from its high light transmission and good outdoor weathering properties It is also a useful moulding material for applications where good appearance, reasonable toughness and rigidity are requirements which are considered to justify the extra cost of the polymer as compared with the large tonnage plastics

For many years the market growth for poly(methy1 methacrylate) was much lower than for other major thermoplastics For example, UK production in 1950 was about the same as that for polystyrene, in 1965 (when the first edition of this book was being completed) it was about 40% and by the end of the 1970s it was down to about 10% There was, however, an upsurge in the late 1980s and early 1990s and world production capacity was estimated at 1.7 X 106t.p.a in 1996 This is about 17% of the capacity for polystyrene During the late 1990s there was

a considerable capacity build-up in Asia and already by 1996 this area claimed about 38% of global capacity followed by America with 34% and Europe 28%

While the overall market is roughly divided between mouldings and sheet products extruded sheet is making inroads into the cast sheet market and in 1997 in the USA

it was estimated that less than 25% of PMMA products were produced from cast (mainly sheet) materials In Western Europe the market has been assessed at auto applications 30%, illumination engineering 20-25%, building industry 15%,

optical industry 10-15%, household goods 8-lo%, and other 15%

The material is eminently suitable for display signs, illuminated and non- illuminated, and for both internal and external use The properties of importance here are weatherability, the variety of techniques possible which enable a wide range of signs to be produced and, in some cases, transparency

In lighting fittings poly(methy1 methacrylate) finds an important outlet Street lamp housings originally shaped from sheet are now injection moulded Ceiling lighting for railway stations, school rooms, factories and offices frequently incorporate poly(methy1 methacrylate) housings In many of these applications opalescent material is used which is effective in diffusing the light source Poly(methy1 methacrylate) is the standard material for automobile rear lamp housings

The methacrylic polymer remains a useful glazing material In aircraft applications it is used extensively on aircraft which fly at speeds less than Mach

1.0 They form the familar ‘bubble’ body of many helicopters On land, acrylic sheet is useful for coach roof lights, motor cycle windscreens and in do-it yourself ‘cabins’ for tractors and earth-moving equipment Injection mouldings are frequently used for plaques on the centre of steering wheels and on some fascia panelling

Transparent guards for foodstuffs, machines and even baby incubators may be fabricated simply from acrylic sheet It should, however, be pointed out that due

to rather rapid surface deterioration and the lack of ‘sparkle’ the material is not ideally suited as a cover for displayed goods

Acrylic sheet is also employed for many other diverse applications, including baths and wash-basins, which have considerable design versatility, are available

in a wide range of colours, and are cheaper and much lighter than similar products from other materials

Extruded sheet is cheaper than cast sheet but because there is some residual molecular orientation, is somewhat less satisfactory optically and more difficult

to machine On the other hand, no doubt a function of its lower molecular weight,

it may be thermoformed more easily

The energy crisis that began in the 1970s has led to much interest in solar heating Because of its excellent weathering properties, transparency and light weight compared with glass the material is being used for the dome-shaped covers of solar collectors In this application it is important to use a heat-resistant film between the acrylic dome and the absorbing material, both to reduce heat loss and to protect the acrylic material if there is an accumulation of heat due to failure of the liquid circulation in the absorber

In contrast to the above use PMMA sheet has been used as the ‘bed’ in indoor ultraviolet lamp operated solaria Here the ultraviolet radiation is so intense as to require the use of special formulations with adequate ultraviolet resistance

PMMA has not been able to compete in the field of compact discs, the market having gone to the polycarbonates (see Chapter 20) It is, however, suitable for optical data storage using large video discs Large-scale acceptance in the field

of optical fibres has been held back by problems of obtaining material of an acceptable level of purity

As described in the previous section, casting dispersions based on monomer and fine sand are now finding use in high-grade sanitary ware and tiling Decorative plaques are produced by injection moulding poly(methy1 meth- acrylate) and then coating the back of the transparent moulding with a thin coat

of metal by the vacuum deposition technique or with a paint by spraying By suitable masking, more than one metal and more than one colour paint may be used to enhance the appearance These plaques are frequently used in the centre

of car steering wheels, refrigerators and other equipment where an eye-catching motif is considered desirable

Methyl Methacrylate Polymers with Enhanced Impact Resistance 41 3

If the surface of an acrylic sheet, rod or tube is roughened or carved, less light

is internally reflected and the material is often rather brighter at these non- polished surfaces The use of this effect enables highly attractive carvings to be produced Similarly, lettering cut into sheet, particularly fluorescent sheet, becomes ‘lit-up’ and this effect is useful in display signs

The use of acrylic materials for dentures has already been mentioned

Tensile strength (5 mm/min strain rate)

Tension modulus

Impact strength

Notched impact strength

Elongation at break (5 mm/mm strain rate)

Light transmission

Vicat softening point

15.3 METHYL METHACRYLATE POLYMERS WITH ENHANCED IMPACT RESISTANCE AND SOFTENING POINT

MPa

% MPa

An early approach was to use butadiene as the comonomer but the resultant copolymers have largely been used only in latex form in paper and board finishes and are no longer believed to be important

Copolymers of methyl methacrylate and butyl acrylate gave polymers that were somewhat tougher and slightly softer than the homopolymers Materials believed to be of this type were marketed in sheet form by IC1 as Asterite for a short while in the 1960s (the name having been recently revived for another product as described in Section 15.2.6)

Rather more recently Rohm and Haas GmbH have introduced Plexidur plus which is a copolymer of acrylonitrile and methyl methacrylate It is best considered as a glazing material for use in schools, sports halls and vehicles The material also has good clarity, rigidity and surface hardness Some typical properties compared with PMMA are given in Table 15.2

Following the success in blending rubbery materials into polystyrene, styrene- acrylonitrile and PVC materials to produce tough thermoplastics the concept has been used to produce high-impact PMMA-type moulding compounds These are two-phase materials in which the glassy phase consists of poly(methy1 methacrylate) and the rubbery phase an acrylate polymer, usually poly(buty1 acrylate) Commercial materials of the type include Diakon MX (ICI), Oroglas

Table 15.2 Some properties of a methyl methacrylate-acrylonitrile copolymer compared with a general purpose poly(methy1 methacvlate) compound at 23°C and 50% R.H (German DIN tests)

I

I

MMA-ACN copolymer

Nitrile Resins 415

DR (Rohm and Haas) and Plex 8535-F (Rohm GmbH) Some typical properties

of these materials compared with straight PMMA and with the competitive ABS and ASA polymers (discussed in Chapter 16) are given in Table 15.3

In comparison with the styrene-based and better known ABS and ASA materials the high-impact methacrylates have generally lower values for mechanical properties such as tensile strength, impact strength and modulus However, long-term weathering tests show the marked superiority of the methacrylates over ABS and even ASA materials to degradation In a typical test the impact strength of unnotched high-impact PMMA rods was about sixfold that

of both ABS and ASA materials

Over the years many attempts have been made to produce commercial acrylic polymers with a higher softening point than PMMA The usual approach was to copolymerise MMA with a second monomer such as maleic anhydride or an

N-substituted maleimide which gave homopolymers with a higher Tg than

PMMA In this way copolymers with Vicat softening points as high as 135°C could be obtained

In the early 1990s attention appeared to be focusing on the imidisation of acrylic polymers with primary amines

2 C H , O H

As might be expected from a consideration of the factors discussed in Section

4.2, the imidisation process will stiffen the polymer chain and hence enhance Tg

and thus softening points Hence Vicat softening points (by Procedure B) may be

as high as 175°C The modulus of elasticity is also about 50% greater than that

of PMMa at 4300MPa, whilst with carbon fibre reinforcement this rises to

25 000 MPa The polymer is clear (90% transparent) and colourless

Such materials, known as poly(methy1 methacrylimides) or PMMI, are marketed by Rohm and Haas in the USA as Kamex, and there is a small production by Rohm in Europe, where the product is marketed as Pleximid Hard-coated poly(methy1 methacrylimide) sun-roofs have already been specified for American sports cars, whilst the polymer might be expected to make some inroads into the polycarbonate market, with one specific target being auto headlamp diffusers

15.4 NITRILE RESINS

Poly(acrylonitri1e) has found little use as a plastics material because it softens only slightly below its decomposition temperature of about 300°C In addition it does not dissolve in its monomer so it cannot be shaped by bulk casting It will,

however, dissolve in solvents such as dimethylformamide and tetramethylene- sulphone In consequence poly(acrylonitri1e) and closely related copolymers have found wide use as fibres (e.g Orlon, Acrilan)

Copolymers of acrylonitrile and vinylidene chloride have been used for many years to produce films of low gas permeability, often as a coating on another material Styrene-acrylonitrile with styrene as the predominant free monomer (SAN polymers) has also been available for a long time In the 1970s materials were produced which aimed to provide a compromise between the very low gas permeability of poly(viny1idene chloride) and poly(acrylonitri1e) with the processability of polystyrene or SAN polymers (discussed more fully in Chapter

16) These became known as nitrile resins

Table 15.4 illustrates that though the nitrile resins had a gas permeability much higher than has poly(acrylonitri1e) the figures for oxygen and carbon dioxide are much lower than for other thermoplastics used for packaging

Poly( acrylonitrile) Nitrile resins Poly(viny1idene chloride) Poly(viny1 chloride) High-density polyethylene

Table 15.4 Permeability ( P ) of nitrile resins compared

with other polymers

The common feature of these materials was that all contained a high proportion of acrylonitrile or methacrylonitrile The Vistron product, Barex 210,

for example was said to be produced by radical graft copolymerisation of 73-77 parts acrylonitrile and 23-27 parts by weight of methyl acrylate in the presence

of a 8-10 parts of a butadiene-acrylonitrile rubber (Nitrile rubber) The Du Pont product NR-16 was prepared by graft polymerisation of styrene and acrylonitrile

in the presence of styrene-butadiene copolymer The Monsanto polymer Lopac was a copolymer of 28-34 parts styrene and 66-72 parts of a second monomer variously reported as acrylonitrile and methacrylonitrile This polymer contained

no rubbery component

The main interest in these materials lay in their potential as beverage containers although other suggested uses included such, presumably, diverse materials as barbecue sauces, pesticides and embalming fluids However, in 1977

the US Food and Drugs Administration proposed a ban on these materials for beverage applications and suggested stringent levels of allowable acrylonitrile residual monomer migration This led to companies withdrawing from manu- facture of these resins Shortly afterwards this particular market was penetrated

by polyester resins of the poly(ethy1ene terephthalate) type (see Chapters 21 and